The present invention relates to ring lasers, and particularly to an all-fiber Brillouin ring laser, having a sub-milliwatt pump threshold power.
Present, commercially available, ring lasers typically comprise three or more mirrors, positioned in a medium, such as helium neon gas, and oriented to form a ring-shaped cavity for guiding counter-propagating waves therethrough. Rotation of the ring-shaped cavity causes each of the counter-propagating waves to experience a frequency shift, proportional to the rotation rate. By measuring the frequency difference between the counter-propagating waves, the ring laser provides an indication of rotation rate, and thus, may be used as a highly accurate rotation sensor, e.g., for inertial navigation. One common problem with these ring lasers is that counter-propagating waves tend to become frequency locked, and insensitive to rotation. Such frequency locking may be prevented by continuously dithering (mechanically vibrating) the laser, however, the need for a mechanical dithering system tends to defeat the principal purpose of the ring laser, viz., providing a reliable, accurate, rotation sensor without moving parts. The frequency locking problem may be caused by the fact that the laser's gain is bidirectional, that is, the same collection of HeNe atoms are utilized for stimulated emission of both counter-propagating waves. Accordingly, it is believed that frequency locking may be avoided by utilizing a laser in which the gain is unidirectional. One type of laser having such unidirectional gain is a Brillouin fiber ring laser, in which energy for stimulated Brillouin emission is provided by a pump light wave which counter-propagates relative to the Brillouin wave.
Prior art Brillouin fiber lasers are typically lossy, hybrid devices, in which the resonant laser cavity is formed from both fiber optic and bulk optic components. For example, lenses, mirrors, and beam splitters are commonly coupled to long lengths of single-mode fiber. Maintaining alignment of these components is difficult, particularly if they are subjected to shock or vibration. Further, the round trip losses of light circulating through the laser's resonant cavity are quite high, for example, on the order of 70%. Consequently, high threshold pump power, e.g., on the order of 100 mW is required to achieve Brillouin gain. It is believed that even the most carefully constructed prior art Brillouin fiber lasers would require threshold powers of more than 10 mW.
Prior art Brillouin lasers have, therefore, commonly utilized high power, single frequency lasers, e.g., argon gas lasers, for the pump. Such a laser, however, has inherently poor stability, in terms of frequency drift, unless special stabilization techniques are employed. Further, the coherence length of such lasers is relatively short, and therefore, the purity of the single frequency light is relatively poor.
Accordingly, there is a need in the art for an improved Brillouin laser having low round trip cavity losses, so that a highly stable, low power, long coherence length, single frequency laser, such as a helium neon laser, can be used as a pump.